folder_block_ops: fix small nits in dir write ops
folder_block_ops: fix small nits in dir write ops

Suggested by jzila.

Issue: #901

keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/kbfs